Once again the new drive is a member of Western Digital's Caviar family, which is the backbone of the company's desktop products. Caviar consists of a series of mainstream hard drives, RAID Edition models that have been designed for 24/7 operation, and the Special Edition drives. Drives in this last category are usually equipped with more cache memory than the regular drives.
The WD Caviar Special Edition 16 comes with 16 MB of buffer memory. This is not particularly new - Toshiba has had 2.5" drives with 16 MB for quite some time, and Maxtor was the first to introduce 16 MB to the desktop hard drive space. WD is getting into the game, as they continue to deploy larger cache sizes in their latest drive models.
The rotational speed of the drive is still 7,200 RPM, a figure that is not likely to change for mainstream drives any time soon. We do, however, expect Western Digital to upgrade the 10,000 RPM Raptor drive family with faster and higher capacity models not too long from now.
You can find the WD4000KD on WD's website by looking for the highest performance desktop hard drive with Serial ATA/150 interface. The drive is not yet available with a Serial ATA II interface; if you insist on that - and we don't see any good reason why you should - you will have to wait a while longer.